Separators for use in electrochemical devices, in particular in secondary batteries, mainly serve to physically and electrically separating the anode from the cathode of the electrochemical cell, while permitting electrolyte ions to flow there through, and must be thermally stable during operation of the same.
Additionally, performance attributes of these electrochemical devices, such as cycle life and power, can be significantly affected by the choice of the separator.
Separators may be made of polymer materials which are rendered porous or of fibrous or particulate materials including glass fibers, mineral fibers such as asbestos, ceramics, synthetic polymeric fibers as well as natural polymeric fibers such as cellulose.
Inorganic filler materials have been long used to fabricate separators having a composite structure, said composite separators comprising a silica or other ceramic filler material distributed in a polymeric binder matrix. These filler materials are produced as finely divided solid particulates and used as a vehicle for introducing porosity into the polymeric binder material used to fabricate the composite separator.
A separator precursor solution is typically formulated as an ink or paste comprising a solid particulate inorganic material dispersed in a solution of a polymer binder in a suitable solvent. The ink solution so obtained is usually applied to a surface of an electrode layer and the solvent is then removed from the solution layer so as to obtain a separator layer which adheres to the electrode.
For instance, composite separators are notably described in EP 0814520 A (IMRA AMERICA INC.) Dec. 29, 1997 and U.S. Pat. No. 8,076,025 (FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V.) Dec. 13, 2011.
In many cases, the composite separator materials contain a very high content of inorganic filler. In some instances, the composite separators so obtained exhibit poor mechanical properties and insufficient strength and ductility to be used as a free standing film.
One particular challenge has been to provide for composite separators with acceptable thickness, high strength and flexibility, especially when coated onto a substrate such as an electrode. In current lithium ion batteries using polymer separators, separators have typically a thickness of from about 20 μm to about 40 μm. When deposited on the electrodes at these thicknesses, a composite separator tends to crack during the removal of the volatile carrier. In general, cracking can be reduced by increasing the polymer content of the composite; however, porosity and so ion conductivity is reduced with increasing polymer content. This loss of ion conductivity renders the separator unusable in batteries. Separators of suitable thickness can be obtained using multiple coating and drying steps; however, multiple processing steps increase costs and introduce variability into the process and do also have thickness limitations, although these are less severe with the multiple coating approach.
The separator must be also insoluble in the electrolyte and must resist corrosion by other components in the electrochemical cell and by reaction products generated within the same.